1. Field of the Invention
The present invention relates to a novel ester useful as a transparentizing agent for production of a transparent paper which is quite suitable as a copying paper and a tracing paper which are mainly used for copying, and further as a master paper which is used in well-known duplicating processes such as a diazo process, a silver salt process and electrophotography. The term "master paper" referred to herein means paper which is repeatedly used as an original for duplication.
2. Description of the Prior Art
Conventional, well-known methods for the preparation of transparent paper can be roughly divided into two methods as in the case of preparation of glassine paper; (1) a method comprising beating cellulosic fibers or passing the highly hydrated fibers under heat and pressure through a supercalender to remove any air bubbles from the paper web, whereby light scattering within the paper is reduced and thus the paper is transparentized, and (2) a method comprising impregnating a substance having an index of refraction approximately equal to that of cellulose, for example, dioctylphthalate, tricresylphosphate, polystyrol resins, styrenemaleic anhydride copolymers, alkyd resins and urea resins, into the paper web.
The former method, however, has the disadvantages that the method causes a reduction in the physical strengths of the paper, particularly the tearing strength, and the method causes a change in the dimension of the paper by humidity, that is, a reduction in the dimensional stability, because of the dense structure of paper, with frequent curling of paper.
Furthermore, in the latter method comprising impregnating a substance having an index of refraction similar to that of the fibers into the paper web, the properties of transparent paper obtained vary markedly depending on the kind of the substance, even if the both indices of refraction are similar to each other.
In general, when a substance which is a solid at room temperature is used, the transparent paper obtained is superior in its suitability for pencil-erasure and correction, and stiffness, but unfortunately is poor in its transparency and ink is easily repelled. On the other hand, when a substance which is a liquid at room temperature is used, the transparent paper obtained is superior in its transparency and ink-receptivity, but it is inferior in its suitability for pencil-erasure and correction, and in addition migration of the liquid substance and changes in the quality of the paper during storage occur.
For this reason, many attempts have been made to overcome these defects but satisfactory substances have not yet been found. Investigations on compounds free from the above-described defects which are suitable for the latter impregnation method have been made and it has now been found that the use of some particular esters among those which are obtained from an acid and an alcohol eliminates most of the above-described defects, thus imparting to papers good transparency, dimensional stability, stiffness, storage stability for a long period and suitability for writing, pencil-erasure, printing and correction.